meson: Fix usage of pkgconfig.generate()

The library that the pkgconfig file is for should be the first
positional argument. The `libraries:` kwarg is for libraries that the
user must also link against, and which meson does not know about (and
hence cannot automatically add to the `Libs:` or `Requires:` section
in the .pc file).

Fixes:
```
subprojects/pixman/meson.build:564: DEPRECATION: Library pixman-1 was
passed to the "libraries" keyword argument of a previous call to
generate() method instead of first positional argument. Adding
pixman-1 to "Requires" field, but this is a deprecated behaviour that
will change in a future version of Meson. Please report the issue if
this warning cannot be avoided in your case.
```